The present invention generally relates to a single-frequency simultaneous transmission/reception type radio communication system which allows transmission and reception of speech signals to be performed substantially simultaneously by using a carrier of a single frequency. More particularly, the present invention is concerned with a radio telephone communication system capable of conducting concurrent transmission/reception communication between a telephone set and a radio telephone or transceiver (hereinafter referred to representatively as the radio transceiver) by interconnecting a communication system of the single-frequency simultaneous transmission/reception type and a telephone line.
Mobile radio communication systems which make it possible to conduct speech communications by using, for example, radio transceivers or the like in the open air tend to be increasingly and widely used for practical purposes, which is naturally accompanied with an increasing demand for the facility capable of connecting the radio transceivers with a telephone line.
In this conjunction, it is however noted that in the mobile radio communication systems in which the radio transceivers or the like are used, there is generally adopted an alternate communication system based on a press-to-talk scheme (transmission/reception alternate switching communication scheme).
Parenthetically, in the communication effected by using the press-to-talk type transceivers known heretofore, a simplex communication mode in which a single carrier frequency is used is generally adopted. In the simplex mode communication, an addressee or destination radio transceiver is necessarily set to the receiving state when a counterpart radio transceiver is in a sending mode with the intent to avoid the radio inference. In other words, it was impossible to allow speech communication to be conducted by simultaneously (or concurrently) sending speech information from both the radio transceivers. That is, the speech communication is carried out in an alternate manner such that a counterpart radio transceiver is forced to wait for the end of speech communication from the addressee transceiver in order for the counterapart radio transceiver to send speech information. Thus, it is safe to say that this system is very inconvenient as a communication facility. On the other hand, in the telephone communication system such as public telephone communication systems, a so-called duplex communication system is adopted which allows both subscribers to exchange speeches substantially simultaneously independent of each other. This type communication may be referred to as the bidirectional concurrent speech communication. In this conjunction, it is noted that an attempt for realizing a bidirectional communication between the radio transceiver and the telephone set by interconnecting the simplex-mode transceiver and the duplex-mode telephone line will necessarily have to be based on the simplex communication mode. In the bidirectional transceiver communication system known heretofore, use of two carriers of mutually different frequencies is indispensable, which is not preferable from the standpoint of radio wave utilization efficiency.
In recent years, a single-frequency simultaneous transmission/reception type communication system which enables bidirectional communication by using a same carrier frequency for both reception and transmission has been developed and is actually used for practical applications. With the single-frequency simultaneous transmission/reception type communication system, speech can be transferred without the need for an intervening of switching (or change-over) operation as required in the press-to-talk system regardless of the use of a single carrier frequency.
Parenthetically, a radio communication system in which transmission and reception can be performed substantially on a real-time basis by using a single frequency is disclosed in the specification of PCT Application WO 91/02414 published on Feb. 21, 1991.
Furthermore, a radio transceiver of the single-frequency simultaneous transmission/reception type which is destined for installation on a motor vehicle or automobile (hereinafter referred to simply as the car) is proposed in an article entitled "SIMULTANEOUS TRANSMITTING AND RECEIVING METHOD MOBILE RADIO USING A SINGLE FREQUENCY": Proceedings of 1992 IEICE Spring Conference, Paper No. B-769, (Mar. 15, 1992), pp. 3-336.
For a better understanding of the background techniques of the present invention, an example of the single-frequency simultaneous reception/transmission type communication system will be explained by reference to FIG. 8 of the accompanying drawings. Referring to the figure, voice signals a inputted successively undergo temporal compression (time-base compression) at every interval t to be thereby converted to a modulated signal b, wherein empty or available time accrued in the interval or period t owing to the compression processing mentioned above can be allocated to the reception. More specifically, during the reception interval thus made available, the signals incoming from the counterpart or addressee apparatus and undergone the temporal compression in the same manner as mentioned above can be received. The continuous reception voice or speech output can be obtained by decompressing or expanding twice as long as the received voice signal. Change-over of the transmission/reception operations is controlled by using a transmission/reception switching control signal c having a predetermined duration of e.g. 400 milliseconds, to thereby repeat alternately transmission or sending operation and receiving operation each of e.g. 200 milliseconds on a time division basis. In this way, the simultaneous single-frequency bidirectional communications can be carried out.
In this connection, the timing for the temporal compression/expansion is controlled or managed by making use of a synchronizing signal sent out at every transmission interval, as shown at (b) in FIG. 8.
More specifically, the station (radio transceiver) which started the transmission takes the initiative in sending out the modulated signal (b). On the other hand, the station having received the modulated signal (b) detects the synchronizing signal contained in the modulated signal (b) to thereby control the switching between the transmission and the reception. Subsequently, the transmission and the reception are alternately changed over repetitively by transferring the synchronizing signal. The control to this end is generally performed with the aid of a microcomputer.
In the radio transceiver system of the single-frequency simultaneous transmission/reception type described above, no consideration is paid to the facility for connecting the radio transceiver to the telephone line. Consequently, it is impossible to connect the radio transceiver to the telephone line for allowing communication between the radio transceiver and a telephone set, because the latter cannot directly send out any radio waves to the radio transceiver.
Such being the circumstances, it is required to equip a telephone set with a radio transceiver in order to make it possible to conduct speech communication between the telephone set and other radio transceiver.
In the radio transceiver system of the single-frequency simultaneous transmission/reception type, a voice signal generated through a microphone is divided into signal segments, each of a predetermined minute period of e.g. 400 msec., whereon the signals resulting from the division undergo temporal or time-base compression and a carrier is then modulated with the compressed signal to be subsequently sent out. The counterpart or addressee radio transceiver expands the compressed signal as received to the original length for reproduction of the voice signal. On the other hand, when communication is to be started from a terminal station, a connection procedure must be taken for calling the counterpart or addressee by sending a dial signal corresponding to the identification (ID) number of the counterpart or addressee. For generating the dial signal, there is usually adopted a DTMF (Dural-Tone Multifrequency) signaling or dial-pulse signaling scheme. In this conjunction, it should be mentioned that when the dial signal of relatively low transfer rate is transmitted from the radio transceiver of the single-frequency simultaneous transmission/reception type, the dial signal is likely to be divided with a certain rate, which undesirably involves error in the dial signal as received, leading to occurrence of erroneous connection.
The problem mentioned above can certainly be coped with by modulating the dial signal with a MSK (Minimum Shift Keying) signal which can ensure a high transfer rate on the order of 2,400 bps. In that case, the dial signal can be transmitted during the minute period resulting from the division mentioned previously without substantially incurring any possibility of the MSK dial signal being fragmented. However, when the MSK dial signal is to be transmitted by using the radio transceiver of the single-frequency simultaneous transmission/reception type, the MSK dial signal equally undergoes the temporal compression by a factor of 1/2, as mentioned previously. This means that the compressed dial signal has a frequency which is about twice as high as that of the original dial signal. By way of example, the original MSK dial signal of 2,400 Hz will have a frequency of 4,800 Hz or more after the temporal or time-base compression.
By contrast, the frequency band of a radio transmission channel is usually about 3 kHz. For this reason, in the case of the single-frequency simultaneous transmission/reception communication system in which the signal is temporally compressed by a factor of 1/2, the frequency band of the original signal is decreased by about one half (1.5 kHz). Consequently, the aforementioned MSK modulated signal, i.e., the dial signal cannot be used intact. Thus, realization of the bidirectional concurrent speech communication between a telephone set and radio transceiver of the single-frequency simultaneous transmission/reception type encounters a great difficulty or obstacles which cannot be coped with by using the techniques known heretofore.